This application requests support for an investigation of the natural products chemistry of freshwater filamentous fungi. These rarely-studied fungi are ecologically, morphologically, and taxonomically distinctive, and they commonly display antagonistic effects against other fungi. Like our previous and productive chemical studies of sclerotial and coprophilous fungi, this investigation will employ a systematic, ecology-based approach to organism selection that represents a departure from traditional random microbial screening programs. Based on the propensity of these fungi to show antagonistic effects toward other fungi, together with the increasingly urgent need for new treatments to fight opportunistic fungal infections in humans, efforts will focus on a search for new metabolites with antifungal effects. However, pure compounds will also be tested for other activities through arrangements with a major pharmaceutical company, and by submission to the NCI's antitumor screens. The proposed work involves a collaboration between the PI's group and that of Prof. Carol Shearer, one of the world's foremost experts in aquatic mycology. Preliminary studies of selected aquatic isolates have already afforded 37 new bioactive agents, including some with unusual structures. These initial results provide evidence that this unique, virtually unexplored group of fungi shows considerable promise as a source of novel bioactive natural products, and clearly demonstrate the ability of the research team to perform the proposed studies. Many isolates that exhibit antifungal effects in direct competition assays are available for immediate investigation, and there are no literature reports of prior chemical studies of any of these organisms. Fungal isolates will be cultivated under both liquid and solid-substrate growth conditions, and extracts from these cultures will be subjected to antifungal assays to prioritize cultures for chemical study. Active components will be isolated through bioassay-guided fractionation, and their structures will be determined using spectroscopic methods. While chemical studies progress, further freshwater species will be isolated, identified, and screened for antifungal activity, thereby providing a continuing stream of new isolates for investigation.